Squire's theorem
In fluid dynamics, Squire's theorem states that of all the perturbations that may be applied to a shear flow (i.e. a velocity field of the form U = ( U ( z ) , 0 , 0 ) {\displaystyle \mathbf {U} =(U(z),0,0)} ), the perturbations which are least stable are two-dimensional, i.e.